feat(client): resolve type errors in impersonation storage service

apps/client/src/app/components/header/header.component.ts

@@ -101,7 +101,7 @@ export class GfHeaderComponent implements OnChanges {
   protected hasPermissionToAccessAssistant: boolean;
   protected hasPermissionToAccessFearAndGreedIndex: boolean;
   protected hasPermissionToCreateUser: boolean;
-  protected impersonationId: string;
+  protected impersonationId: string | null;
   protected readonly internalRoutes = internalRoutes;
   protected isMenuOpen: boolean;
   protected readonly routeAbout = publicRoutes.about.path;

apps/client/src/app/services/impersonation-storage.service.ts

@@ -7,11 +7,11 @@ export const IMPERSONATION_KEY = 'impersonationId';
   providedIn: 'root'
 })
 export class ImpersonationStorageService {
-  private hasImpersonationChangeSubject = new BehaviorSubject<string>(
+  private hasImpersonationChangeSubject = new BehaviorSubject<string | null>(
     this.getId()
   );
 
-  public getId(): string {
+  public getId(): string | null {
     return window.localStorage.getItem(IMPERSONATION_KEY);
   }